Flow regulation of collecting duct endothelin-1 production

Collecting duct (CD) endothelin-1 (ET-1) is an important autocrine inhibitor of CD Na(+) reabsorption. Salt loading is thought to increase CD ET-1 production; however, definitive evidence of this, as well as understanding of the mechanisms transducing this effect, is lacking. Tubule fluid flow increases in response to Na(+) loading; hence, we studied flow modulation of CD ET-1 production. Three days of a high-salt diet increased mouse and rat inner medullary CD (IMCD) ET-1 mRNA expression. Acute furosemide infusion increased urinary ET-1 excretion in anesthetized rats. Primary cultures of mouse or rat IMCD detached in response to flow using a closed perfusion chamber, consequently a CD cell line (mpkCCDcl4) was examined. Flow increased ET-1 mRNA at shear stress rates exceeding 1 dyne/cm(2), with the maximal effect seen between 2 and 10 dyne/cm(2). Induction of ET-1 mRNA was first evident after 1 h, and most apparent after 2 h, of flow. Inhibition of calmodulin or dihydropyridine-sensitive Ca(2+) channels did not alter the flow response; however, chelation of intracellular Ca(2+) or removal of extracellular Ca(2+) largely prevented flow-stimulated ET-1 mRNA accumulation. Downregulation of protein kinase C (PKC) using phorbol 12-myristate 13-acetate, or PKC inhibition with calphostin C, markedly reduced flow-stimulated ET-1 mRNA levels. Flow-stimulated ET-1 mRNA accumulation was abolished by inhibition of phospholipase C (PLC). Taken together, these data indicate that flow increases CD ET-1 production and this is dependent on extracellular and intracellular Ca(2+), PKC, and PLC. These studies suggest a novel pathway for coupling alterations in extracellular fluid volume to CD ET-1 production and ultimately control of CD Na(+) reabsorption.     

Role of endothelin-1 in regulation of the postnatal intestinal circulation

Newborn intestine is uniquely prone to vasoconstriction in response to a wide variety of perturbations. To test the hypothesis that endothelin (ET)-1 is an important factor in this process, we determined the effects of exogenous ET-1 administration and blockade of endogenous ET-1 in vivo and in vitro in 3- and 35-day-old swine. Intramesenteric artery administration of exogenous ET-1 to vascularly isolated in vivo gut loops (10(-9) M/kg bolus) caused vasoconstriction and reduced gut O(2) uptake similarly in these age groups. Selective blockade of ET(A) or ET(B) receptors with BQ-610 or BQ-788, respectively, in vascularly isolated in vivo gut loops had no effect on gut vascular resistance or O(2) uptake in either age group; within in vitro gut loops, BQ-610 significantly increased vasoconstriction when perfusion pressure was reduced below baseline, but only in 3-day-old animals; i.e., it impaired the autoregulatory response to perfusion pressure reduction. Exogenous ET-1 significantly decreased capillary perfusion within in vitro gut loops, as evidenced by a decrease in capillary filtration coefficient, but only in 3-day-old animals; furthermore, blockade of endogenous ET-1 activity with BQ-610 significantly enhanced capillary filtration coefficient in 3-day-old animals and increased O(2) extraction ratio. ET-1 did not depress intestinal metabolic rate, as evidenced by its effect on the O(2) uptake-blood flow relationship; it did compromise tissue oxygenation because of its effects on intestinal O(2) transport. ET-1 concentration in mesenteric venous effluent exceeded arterial concentration, but only in 3-day-old intestine, suggesting production of ET-1 by newborn intestine. We conclude that ET-1 exerts an age-dependent effect on intestinal hemodynamics in postnatal intestine, having a greater impact in 3- than in 35-day-old intestine.     

Effects of atrial natriuretic factor on blood pressure and the renin-angiotensin-aldosterone system

Atrial natriuretic factor (ANF) antagonizes vasoconstriction induced by numerous smooth muscle agonists and also lowers blood pressure in intact animals. ANF has particularly marked relaxant effects on angiotensin II-contracted vessels in vitro. Sensitivity to the blood pressure-lowering effect of ANF in vivo appears to be enhanced in renin-dependent models of renovascular hypertension compared with other experimental hypertensive models. The depressor action of low, possibly physiological doses of ANF in two-kidney, one-clip Goldblatt rats is due to a decrease in total peripheral resistance. On the other hand, high doses of ANF can lower cardiac output, particularly in volume-expanded models such as deoxycorticosterone-salt hypertension. ANF markedly inhibits renin secretion in intact animals, probably via increased glomerular filtration rate and load of sodium chloride to the macula densa. This effect is masked when renal perfusion is impaired (e.g., via unilateral renal artery constriction), in which case ANF may stimulate renin secretion slightly. ANF also reduces plasma aldosterone in vivo and inhibits basal and agonist-induced aldosterone release from isolated adrenal cortical cells. This effect appears to be especially marked for angiotensin-induced aldosterone production in vivo and in vitro. These findings indicate that ANF has potentially important interactions with the renin-angiotensin-aldosterone system and suggest a role for ANF in the homeostatic control of blood pressure as well as of extracellular fluid volume.     

Na delivery and ENaC mediate flow regulation of collecting duct endothelin-1 production

Collecting duct (CD) endothelin-1 (ET-1) is an important autocrine inhibitor of Na and water transport. CD ET-1 production is stimulated by extracellular fluid volume expansion and tubule fluid flow, suggesting a mechanism coupling CD Na delivery and ET-1 synthesis. A mouse cortical CD cell line, mpkCCDc14, was subjected to static or flow conditions for 2 h at 2 dyn/cm(2), followed by determination of ET-1 mRNA content. Flow with 300 mosmol/l NaCl increased ET-1 mRNA to 65% above that observed under static conditions. Increasing perfusate osmolarity to 450 mosmol/l with NaCl or Na acetate increased ET-1 mRNA to ～184% compared with no flow, which was not observed when osmolarity was increased using mannitol or urea. Reducing Na concentration to 150 mosmol/l while maintaining total osmolarity at 300 mosmol/l with urea or mannitol decreased the flow response. Inhibition of epithelial Na channel (ENaC) with amiloride or benzamil abolished the flow response, suggesting involvement of ENaC in flow-regulated ET-1 synthesis. Aldosterone almost doubled the flow response. Since Ca(2+) enhances CD ET-1 production, the involvement of plasma membrane and mitochondrial Na/Ca(2+) exchangers (NCX) was assessed. SEA0400 and KB-R7943, plasma membrane NCX inhibitors, did not affect the flow response. However, CGP37157, a mitochondrial NCX inhibitor, abolished the response. In summary, the current study indicates that increased Na delivery, leading to ENaC-mediated Na entry and mitochondrial NCX activity, is involved in flow-stimulated CD ET-1 synthesis. This constitutes the first report of either ENaC or mitochondrial NCX regulation of an autocrine factor in any biologic system.     

Role of heme oxygenase-1 in the regulation of blood pressure and cardiac function

Heme oxygenase (HO) is a cytoprotective enzyme that degrades heme (a potent oxidant) to generate carbon monoxide (a vasodilatory gas that has anti-inflammatory properties), bilirubin (an antioxidant derived from biliverdin), and iron (sequestered by ferritin). Because of the properties of inducible HO (HO-1) and its products, we hypothesized that HO-1 would play an important role in the regulation of cardiovascular function. In this article, we will review the role of HO-1 in the regulation of blood pressure and cardiac function and highlight previous studies from our laboratory using gene deletion and gene overexpression transgenic approaches in mice. These studies will include the investigation of HO-1 in the setting of hypertension (renovascular), hypotension (endotoxemia), and ischemia/reperfusion injury (heart). In a chronic renovascular hypertension model, hypertension, cardiac hypertrophy, acute renal failure, and acute mortality induced by one kidney-one clip surgery were more severe in HO-1-null mice. In addition, HO-1-null mice with endotoxemia had earlier resolution of hypotension, yet the mortality and the incidence of end-organ damage were higher in the absence of HO-1. In contrast, mice with cardiac-specific overexpression of HO-1 had an improvement in cardiac function, smaller myocardial infarctions, and reduced inflammatory and oxidative damage after coronary artery ligation and reperfusion. Taken together, these studies suggest that an absence of HO-1 has detrimental consequences, whereas overexpression of HO-1 plays a protective role in hypoperfusion and ischemia/reperfusion injury.     

Chronobiology of the renin-angiotensin-aldosterone system in dogs: relation to blood pressure and renal physiology

The renin-angiotensin-aldosterone system (RAAS) plays a pivotal role in the regulation of blood pressure and volume homeostasis. Its contribution to the development of cardiovascular diseases has long been recognized. Extensive literature has shown that peptides of the RAAS oscillate with a circadian periodicity in humans, under strong influence of posture, sleep, and age. Although observations of time-variant changes in the renin cascade are available in dogs, no detailed chronobiological investigation has been conducted so far. The present studies were designed to explore the circadian variations of plasma renin activity (RA) and urinary aldosterone-to-creatinine ratio (U_A:C) in relation to blood pressure (BP), sodium (U_Na, U_Na,fe), and potassium (U_K, U_K,fe) renal handling. Data derived from intensive blood and urine sampling, as well as continuous BP monitoring, were collected throughout a 24-h time period, and analyzed by means of nonlinear mixed-effects models. Differences between the geometric means of day and night observations were compared by parametric statistics. Our results show that variables of the renin cascade, BP, and urinary electrolytes oscillate with significant day-night differences in dogs. An approximately 2-fold (1.6–3.2-fold) change between the average day and night measurements was found for RA (p?<?0.001), U_A:C (p?=?0.01), U_K,fe (p?=?0.01), and U_Na (p?=?0.007). Circadian variations in BP, albeit small (less than 10?mm Hg), were statistically significant (p?<?0.01) and supported by the model-based analysis. For all variables but U_Na and U_Na,fe, the levels were higher at night than during the day. The data also indicate that blood pressure oscillates in parallel to the RAAS, such that, as opposed to healthy humans, BP does not drop at night in dogs. The postprandial decrease in RA is assumed to be related to body fluid volume expansion secondary to water and sodium intake, whereas the reduction of U_A:C reflects aldosterone-stimulated secretion by the renin-angiotensin II pathway. U_Na and U_Na,fe peaked in the afternoon, about 7–8?h after food intake, which is consistent with the “impulse-response pattern” of sodium excretion described in previous publications. Finally, U_K and U_K,fe mirrored aldosterone-mediated potassium secretion in the kidney tubules. To describe the circadian variations of the various variables, two different mathematical representations were applied. A cosine model with a fixed 24-h period was found to fit the periodic variations of RA, U_A:C, U_K, U_K,fe, and BP well, whereas changes in U_Na and U_Na,fe were best characterized by a surge model. The use of nonlinear mixed effects allowed estimation of population characteristics that can influence the periodicity of the RAAS. Specifically, sodium intake was found to interact with the tonic and the phasic secretion of renin, suggesting that varying feeding time could also impact the chronobiology of the renin cascade. (Author correspondence: jonathan.mochel@novartis.com)     

Role of bradykinin B1 and B2 receptors in normal blood pressure regulation

With inhibition or absence of the bradykinin B2 receptor (B2R), B1R is upregulated and assumes some of the hemodynamic properties of B2R, indicating that both participate in the maintenance of normal vasoregulation or to development of hypertension. Herein we further evaluate the role of bradykinin in normal blood pressure (BP) regulation and its relationship with other vasoactive factors by selectively blocking its receptors. Six groups of Wistar rats were treated for 3 wk: one control group with vehicle alone, one with concurrent administration of B1R antagonist R-954 (70 microg x kg(-1) x day(-1)) and B2R antagonist HOE-140 (500 microg x kg(-1) x day(-1)), one with R-954 alone, one with HOE 140 alone, one with concurrent administration of both R-954 and HOE-140 plus the angiotensin antagonist losartan (5 mg x kg(-1) x day(-1)), and one with only losartan. BP was measured continuously by radiotelemetry. Only combined administration of B1R and B2R antagonists produced a significant BP increase from a baseline of 107-119 mmHg at end point, which could be partly prevented by losartan and was not associated with change in catecholamines, suggesting no involvement of the sympathoadrenal system. The impact of blockade of bradykinin on other vasoregulating systems was assessed by evaluating gene expression of different vasoactive factors. There was upregulation of the eNOS, AT1 receptor, PGE2 receptor, and tissue kallikrein genes in cardiac and renal tissues, more pronounced when both bradykinin receptors were blocked; significant downregulation of AT2 receptor gene in renal tissues only; and no consistent changes in B1R and B2R genes in either tissue. The results indicate that both B1R and B2R contribute to the maintenance of normal BP, but one can compensate for inhibition of the other, and the chronic inhibition of both leads to significant upregulation in the genes of related vasoactive systems.     

Role of sulfate conjugation of catecholamines in blood pressure regulation

Catecholamine sulfates were found to be inactive at vascular receptor sites. Only at one nonvascular receptor site important for blood pressure (BP) regulation was dopamine-3-O-sulfate found to be an inhibitory modulator of the adrenocortical secretion of aldosterone in vitro. However, sulfation of catecholamines (CA) does play an important role in BP regulation, as shown by the following observations: Sulfoconjugated CA with a long half-life are sometimes better markers of CA release than free CA, which have short half-lives. This is particularly true of dopamine (DA) sulfate because free DA, being rapidly sulfoconjugated, is usually undetectable. This led to the recognition of a previously unsuspected role of DA in paroxysmal hypertension and orthostatic hypotension. Measurements of CA sulfates reveal potentially important storage functions for sulfoconjugation that can rapidly inactivate excessive circulating free CA and so mitigate their cardiovascular impact while building up a pool of conjugated CA. The possibility that free CA can be generated from this pool through deconjugation whenever a need for them arises should be further investigated. Reproducible individual differences in the velocity of the sulfoconjugation of infused free CA can be detected, which suggests a genetic control of certain components of the sulfoconjugating process. These differences in sulfoconjugation probably modulate the cardiovascular action of CA and of some drugs with similar structure that also undergo sulfoconjugation.     

Role of endothelin-1 in stress response in the central nervous system

Endothelin (ET)-1 is a 21-amino acid peptide that induces a variety of biological activities, including vasoconstriction and cell proliferation, and its likely involvement in cardiovascular and other diseases has recently led to broad clinical trials of ET receptor antagonists. ET-1 is widely distributed in the central nervous system (CNS), where it is thought to regulate hormone and neurotransmitter release. Here we show that CNS responses to emotional and physical stressors are differentially affected in heterozygous ET-1-knockout mice, which exhibited diminished aggressive and autonomic responses toward intruders (emotional stressors) but responded to restraint-induced (physical) stress more intensely than wild-type mice. This suggests differing roles of ET-1 in the central pathways mediating responses to different types of stress. Hypothalamic levels of ET-1 and the catecholamine metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) were both increased in wild-type mice subjected to intruder stress, whereas MHPG levels were not significantly affected in ET-1-knockout mice. Furthermore, immunohistochemical analysis showed that ET-1 and tyrosine hydroxylase, an enzyme in the catecholamine synthesis pathway, were colocalized within certain neurons of the hypothalamus and amygdala. Our findings suggest that ET-1 modulates central coordination of stress responses in close association with catecholamine metabolism.     

Dexamethasone stimulates endothelin-1 gene expression in renal collecting duct cells

Aldosterone stimulates the endothelin-1 gene (Edn1) in renal collecting duct (CD) cells by a mechanism involving the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). The goal of the present study was to determine if the synthetic glucocorticoid dexamethasone affected Edn1 gene expression and to characterize GR binding patterns to an element in the Edn1 promoter. Dexamethasone (1μM) induced a 4-fold increase in Edn1 mRNA in mIMCD-3 inner medullary CD cells. Similar results were obtained from cortical collecting duct-derived mpkCCD(c14) cells. RU486 inhibition of GR completely blocked dexamethasone action on Edn1. Similarly, 24h transfection of siRNA against GR reduced Edn1 expression by approximately 50%. However, blockade of MR with either spironolactone or siRNA had little effect on dexamethasone induction of Edn1. Cotransfection of MR and GR siRNAs together had no additive effect compared to GR-siRNA alone. The results indicate that dexamethasone acts on Edn1 exclusively through GR and not MR. DNA affinity purification studies revealed that either dexamethasone or aldosterone resulted in GR binding to the same hormone response element in the Edn1Edn1 promoter. The Edn1 hormone response element contains three important sequence segments. Mutational analysis revealed that one of these segments is particularly important for modulating MR and GR binding to the Edn1 hormone response element.     

Role of atrial natriuretic peptides and neuropeptide Y in blood pressure regulation

Atrial natriuretic peptides (ANP) are released into the circulation in response to enhanced atrial stretching. These peptides not only have diuretic and natriuretic properties, but also exert a relaxing effect on the vasculature. Moreover, they antagonize the contractions induced by norepinephrine and angiotensin II. Neuropeptide Y (NPY) is also a vasoactive peptide. It is widely distributed throughout the central and peripheral nervous systems. NPY is coreleased with norepinephrine by perivascular nerve endings. At high concentrations, this peptide has a direct vasoconstrictor effect. In addition, it enhances the vascular effect of various agonists, including norepinephrine and angiotensin II. Both ANP and NPY have an inhibitory effect on renin secretion. This effect may have important implications for the role of these peptides in cardiovascular regulation.     

Role of endothelin-1 in lung disease

Endothelin-1 (ET-1) is a 21 amino acid peptide with diverse biological activity that has been implicated in numerous diseases. ET-1 is a potent mitogen regulator of smooth muscle tone, and inflammatory mediator that may play a key role in diseases of the airways, pulmonary circulation, and inflammatory lung diseases, both acute and chronic. This review will focus on the biology of ET-1 and its role in lung disease.     

Prevention of endothelin-1-induced increases in blood pressure: role of endogenous CGRP

To determine the role of endothelin-1 (ET-1) and its receptors in the regulation of calcitonin gene-related peptide (CGRP) release, male Wistar rats were divided into six groups and subjected to the following treatments for 1 wk with or without ABT-627 (an ET(A) receptor antagonist, 5 mg.kg(-1).day(-1) in drinking water) or A-192621 (an ET(B)-receptor antagonist, 30 mg.kg(-1).day(-1) by oral gavage): control (Con), ET-1 (5 ng.kg(-1).min(-1) iv), Con + ABT-627, Con + A-192621, ET-1 + ABT-627, and ET-1 + A-192621. Baseline mean arterial pressure (MAP, mmHg) was higher (P < 0.05) in Con + A-192621 (122 +/- 4) and ET-1 + A-192621 (119 +/- 4) groups compared with Con (104 +/- 6), ET1 (106 +/- 3), Con + ABT-627 (104 +/- 3), and ET1 + ABT-627 (100 +/- 3) groups. Intravenous administration of CGRP(8-37) (a CGRP receptor antagonist, 1 mg/kg) increased MAP (P < 0.05) in ET-1 (13 +/- 1), Con + A-192621 (12 +/- 1), and ET-1 + A-192621 (15 +/- 3) groups compared with Con (4 +/- 1), Con-ABT-627 (4 +/- 1), and ET-1 + ABT-627 (5 +/- 1) groups. Plasma CGRP levels (in pg/ml) were increased (P < 0.05) in ET-1 (57.5 +/- 6.1), Con + A-192621 (53.9 +/- 3.4), and ET-1 + A-192621 (60.4 +/- 3.0) groups compared with Con (40.4 +/- 1.6), Con + ABT-627 (40.0 +/- 2.9), and ET-1 + ABT-627 (42.6 +/- 1.9) groups. Plasma ET-1 levels (in pg/ml) were higher (P < 0.05) in ET-1 (2.8 +/- 0.2), ET-1 + ABT-627 (3.2 +/- 0.4), Con + A-192621 (3.3 +/- 0.4), and ET-1 + A-192621 (4.6 +/- 0.3) groups compared with Con (1.1 +/- 0.2) and Con-ABT-627 (1.3 +/- 0.2) groups. Therefore, our data show that ET-1 infusion leads to increased CGRP release via activation of the ET(A) receptor, which plays a compensatory role in preventing ET-1-induced elevation in blood pressure.     

Glucocorticoids in blood pressure regulation

Glucocorticoid excess is associated with hypertension in man and in animals. This type of hypertension is of rapid onset and independent of salt intake; it can be inhibited by glucocorticoid antagonists, such as RU486. In normal rats, RU486 blunts vascular reactivity to norepinephrine and angiotensin II. In normal rats on low-salt diet, it induces a fall in blood pressure of approximately 20 mm Hg, due to decreased vascular resistance. This suggests that in some conditions, endogenous glucocorticoids contribute to the maintenance of blood pressure by enhancing vascular reactivity.     

Role of renin-angiotensin-aldosterone system in salt-sensitive hypertension induced by sensory denervation

To define the role of the renin-angiotensin-aldosterone system in a novel salt-sensitive model, neonatal Wistar rats were given capsaicin (50 mg/kg sc) on the first and second days of life. After weaning, male rats were divided into the following six groups and treated for 3 wk with: control + normal sodium diet (CON-NS), CON + high-sodium diet (CON-HS), CON + HS + spironolactone (50 mg x kg(-1) x day(-1), CON-HS-SP), capsaicin pretreatment + NS (CAP-NS), CAP-HS, and CAP-HS-SP. Radioimmunoassay shows that plasma renin activity (PRA) and plasma aldosterone level (PAL) were suppressed by HS, but they were higher in CAP-HS than in CON-HS and CON-HS-SP (P < 0.05). Both tail-cuff systolic blood pressure and mean arterial pressure were higher in CAP-HS than in all other groups (P < 0.05). Urine water and sodium excretion were increased with HS intake, but they were lower in CAP-HS than in CON-HS (P < 0.05). Western blot did not detect differences in adrenal AT1 receptor content. Therefore, insufficiently suppressed PRA and PAL in response to HS intake by sensory denervation may contribute to increased salt sensitivity and account for effectiveness of spironolactone in lowering blood pressure in this model.     

血管紧张素—（1—7）在大鼠延髓头端腹外侧通过氨基酸类递质调节血压的作用

采用微量注射、微透析、高效液相色谱－荧光测定等技术和方法,观察和 血管紧张素－（1－7）[Ang-(1-7)]在延髓头端腹外侧（RVLM）与氨基酸类递质释放之间的关系,在麻醉大鼠RVLM注射Ang-(1-7)可引起血压升高,同时伴RVLM兴奋性氨基酸（EAA）释放增多;在RVLM注射Ang-(1-7)选择性受体拮抗剂Ang779可引起血压降低,同时伴RVLM EAA释放减少和抑制性氨基酸（IAA）释放增多,Ang-(1-7)的升压作用和Ang779的降压作用均可被相应的氨基酸受体拮抗剂部分阻娄。结果提示,Ang-(1-7)在RVLM的升压效庆可能部分是通过EAA释放增多所致;而Ang779在 RVLM的降压效应可能部分是通过EAA释放减少、IAA释放增多所致。     

Selected zinc metabolism parameters in relation to insulin, renin-angiotensin-aldosterone system, and blood pressure in healthy subjects

The relationship between the renin-angiotensin-aldosterone system and insulin concentration and selected zinc (Zn) metabolism parameters and arterial blood pressure in young healthy subjects of both sexes is presented in this study. The following parameters were measured: systolic and diastolic arterial blood pressure, total and ouabain-dependent efflux rate constants of Zn from lymphocytes, serum and lymphocyte Zn concentrations, serum aldosterone, angiotensin-converting enzyme, insulin, sodium and potassium concentrations, body mass index, and plasma rennin activity. The correlations among these parameters show gender-dependent differences, except for a negative correlation between serum Zn and ouabain-dependent Zn efflux rate constant and the serum level of angiotensin-converting enzyme, and a positive relationship between the total efflux rate constant of Zn from lymphocytes and the serum aldosterone levels, both of which were gender independent. The results led us to conclude that there is a gender-independent functional relation between Zn homeostasis and the renin-angiotensin-aldosterone system. Insulin does not appear to play a significant role in Zn homeostasis.     

Role of the renin-angiotensin system in regulation and autoregulation of renal blood flow

The role for ANG II in renal blood flow (RBF) autoregulation is unsettled. The present study was designed to test the effect of clamping plasma ANG II concentrations ([ANG II]) by simultaneous infusion of the angiotensin-converting enzyme inhibitor captopril and ANG II on RBF autoregulation in halothane-anesthetized Sprague-Dawley rats. Autoregulation was defined as the RBF response to acute changes in renal perfusion pressure (RPP). Regulation was defined as changes in RBF during long-lasting changes in RPP. The results showed that a prolonged reduction of RPP reset the lower limit of autoregulation from 85 +/- 1 to 73 +/- 2 mmHg (P < 0.05) and regulated RBF to a lower level. Reduction of RPP to just above the lower limit of autoregulation (88 mmHg) induced regulation of RBF to a lower level within 10 min. Clamping [ANG II] per se reset the lower limit of autoregulation to 62 +/- 5 mmHg. In this case, reduction in RPP to 50 mmHg did not induce a downregulation of RBF. We conclude that ANG II plays an important role in the resetting of the autoregulation limits. The ability to regulate RBF to a new level as a response to changes in RPP also depends on changes in [ANG II].